The present invention relates to solid-state display devices and means to store and display pixel values and images.
Solid-state displays can be characterized as emissive or non-emissive. An emissive display directly generates light at each pixel and requires power to operate and display information. Liquid crystal displays (LCDs), in contrast, are non-emissive and maintain their state without drawing significant current. (LCDs are non-volatile although power is needed to make their state visible either through back-lighting or ambient light, or to change their state. The switched state is maintained through an applied electrostatic field.) The liquid crystals themselves do not emit light but rather change the polarization of light passing through them. LCDs are thus non-emissive and generally utilize a back-light to make their display visible. A non-volatile display is, by definition, persistent.
Solid state image display devices utilizing light emissive pixels are well known and widely used. Much work has been done to improve the brightness, uniformity, contrast, etc. of the displays so as to make them as pleasing as possible. For example, European Patent Application EP 0 905 673 A1, by Kane et al., published Mar. 31, 1999, entitled xe2x80x9cActive Matrix Display System and a Method for Driving the Samexe2x80x9d and the article entitled xe2x80x9cA Polysilicon Active Matrix Organic Light Emitting Diode Display with Integrated Driversxe2x80x9d by Dawson et al., published in the Society for Information Display Digest, 1998, pages 11-14, describe such efforts. Generally speaking, these devices require power to maintain their information state (they are volatile), and because of charge leakage, can only maintain and display an image for a limited amount of time after which it begins to fade (they are not persistent). The image is then refreshed, that is the image is rewritten into the display device. Refresh circuitry can be complex, require high data rates, and impose a significant financial, power, and size burden on a system. In particular, refreshing a display requires a significant use of system power. The frequency with which the display must be rewritten depends on the persistence of the display (how long it can maintain an acceptable image) and the rate at which the image content changes. If the image content changes more frequently than the rate at which the image fades, there will never be a problem. This is generally the case in video-rate systems. However, in cases where the content changes slowly or where only portions of an image change, a periodic display refresh may be unnecessary for a persistent display. Hence, persistence can be a useful attribute in a display and reduces the system cost and power consumption. For example, a persistent emissive display designed for still images alone may not require periodic refresh capability.
One mechanism used to create non-volatile displays is to integrate non-volatile memory elements directly within the display device. For example, U.S. Pat. No. 5,953,061 issued Sep. 14, 1999 to Biegelsen et al., entitled xe2x80x9cPixel Cells Having Integrated Analog Memories and Arrays Thereofxe2x80x9d describes such a system based on ferro-electric memory designs. This approach, while viable, requires considerable supporting electronic elements to implement and frequently relies on problematic materials and manufacturing processes.
Solid-state image displays are typically organized by address and data controls representing the value of each pixel in the display. The address is converted into a select line (or combination of select lines) controlling an individual pixel and a data line representing the analog value of the pixel. Each pixel is then managed by the Data and Select control lines and incorporates means to store a charge representing the value of the pixel at the pixel site, and a mechanism to emit light from the stored charge. The control mechanisms are generally implemented using transistors and the storage mechanisms through capacitors. U.S. Pat. No. 5,552,678 issued Sep. 3, 1996 to Tang et al., entitled xe2x80x9cAC Drive Scheme for Organic LEDxe2x80x9d describes an AC drive scheme for use with organic LEDs.
FIG. 1 is a schematic block diagram of a typical prior art display pixel in an emissive display. The display element includes a control logic block 42, a charge storage block 44, and a display block 48.
FIG. 2 shows a circuit diagram implementing the block diagram of FIG. 1. for an LED display. In this figure, the pixel is formed on a substrate 10, and includes a control transistor Tc 12, (corresponding to the control logic block 42 in FIG. 1) that stores charge on a storage capacitor Cref 14 (corresponding to charge storage block 44) which is connected to the gate of a display transistor Td 20, which controls current to an LED 22 (corresponding to display block 48 in FIG. 1). The control transistor Tc 12 is responsive to signals applied to control lines (Select 16 and Data 18) and, when active, deposits a charge onto charge storage capacitor Cref 14. Cref 14 then controls the drive transistor Td 20, which controls current to the LED 22. Td 20 is optimized to effectively drive the LED 22; Tc 12 to charge the storage capacitor 14 and respond to the control lines 16 and 18. To perform these tasks, both transistors 12 and 20 tend to be large; Tc 12 to provide fast switching time and Td 20 to provide the maximum current (and brightness) through the LED 22.
The persistence of the display is directly related to the length of time that the storage capacitor 14 can maintain its charge. There are three basic mechanisms through which this charge can dissipate. The first is directly across the capacitor 24 (leakage) and will be affected by the materials and structures used to implement the capacitor in the circuit. Second, charge 26 is dissipated to drive the display transistor 20. Third, charge 28 can leak back through the control transistor 12. These leakage paths are illustrated with the curved arrows 26 and 28 in FIG. 2. Leakage through the capacitor 14 itself is exacerbated by material impurities; leakage back through control transistor Tc 12 is attributed to source-to-drain and source-to-gate leakage; and leakage through display transistor Td 20 by gate-to-source leakage. The leakage through the transistors is greater for larger transistors.
Generally, each display device uses electronic elements, transistors, capacitors, and the like that are optimized to the manufacturing process and the task to which the elements are put. The traditional arrangement and size of storage and emitter drivers decreases the persistence of the display. This, in turn, imposes system costs on any practical imaging system by forcing periodic refresh requirements. These system costs can include design effort, manufacturing costs, complexity, performance, reduced system reliability, and power. There is a need therefore for an improved persistent emissive display that is less costly to manufacture, has a simpler design and exhibits improved performance over the prior art devices.
The above noted need is met according to the present invention by providing a persistent emissive display device, including: a light emitting element; a drive circuit connected to the light emitting element, the drive circuit including a transistor having a gate for controlling the power applied to the light emitting element; a storage capacitor connected to the gate of the drive circuit transistor; a control circuit for depositing charge on the storage capacitor; and a circuit element for reducing charge leakage from the storage capacitor, whereby attributes of the display including the persistence of the display, switching speed, and power can be optimized for a given application.